PROJECT SUMMARY Spinal cord injury (SCI) impairs sensory transmission leads to chronic, debilitating neuropathic pain. Chronic pain afflicts over 100 million Americans and creates an enormous burden on US health care systems, costing over half a trillion dollars annually according to a recent report from the Institute of Medicine. While our understanding of the molecular basis underlying the development of chronic pain has improved, the available therapeutics provide limited relief. In the proposed administrative supplement, we will examine the peripheral immune and inflammatory response. Secondary inflammation in response to SCI is a series of temporally ordered events- an acute, transient upregulation of chemokines, followed by the recruitment of monocytes/ macrophages and generation of an inflammatory environment at the lesion site in the spinal cord, but also surrounding primary nociceptors in the dorsal root ganglia. These events precede neuropathic pain development. Work on our funded R01 has shown that at chronic time points after SCI, macrophage presence in the dorsal root ganglia correlates with neuropathic pain. We will extend the currently funded experiments by conducting experiments to better understand 1) whether the phenotype of macrophages that infiltrate the dorsal root ganglia is different than those that persist chronically after SCI and 2) how manipulation of macrophage phenotype affects nociceptor activity and pain development. Moreover, a large portion of this application is dedicated to rigorous validation of the potential of the macrophage as a target for future interventions. The proposed experiments include: 1) multiple species of animals, 2) inclusion of several tests for aberrant pain behavior, 3) consider and measure sex as a potential biological variable, and 4) attend to the potential of off-target effects of macrophage manipulation on other functional outcomes like locomotion and spasticity. Understanding the temporal response of these immune cells and how they affect nociceptor activity and pain behavior will guide future efforts to optimize the treatment of SCI-induced pain.